The shrinking temperature has as much to do with the speed the iron moves across the fabric as it does with the temperature of the iron. If you move quickly you transfer much less heat to the cloth than if you move slowly.

I use a couple different irons when I do this, a small iron for those tight areas near the ends, while it doesn't get as hot as the bigger iron it reaches spots the bigger one has trouble getting to.

You really don't want the fabric "drum" tight, it's too prone to tearing if you hit something with it and shrunk Dacron (trade name) can actually crush a frame if it's shrunk too tight. Much has to do with how snug you get the fabric onto the frame before you shrink it, it only shrinks so much. If it's pretty tight un-shrunk you can get it tighter when shrinking than if it's fairly loose when it's first put on the frame.

Bill H.